Where there is rolling contact between objects under a load, there may be the development of cracks. This may be a problem in bearings, and in the wheels or rails of railways. In the context of rails it has been found that such cracking, which may be referred to as head checking, may sometimes develop within a year of laying a new rail yet in other situations no such cracks develop over several decades. It would clearly be desirable to be able to monitor a material in situ to ascertain if rolling contact fatigue is developing, and particularly if the formation of cracks is incipient. Rolling contact fatigue may be presumed to arise from the stresses imposed on the material in use—by the passage of trains, in the case of a rail—but the exact mechanism for its development is not clear. The stresses in structures such as rails arise from various causes including changes of temperature, and the loads and pressures due to use. There may also be residual stresses arising from the fabrication of the structure or device, and any bending that the structure or device was subjected to during construction; the residual stresses arising from fabrication will also be affected by any stress-relieving heat treatment. As regards ferromagnetic materials, a variety of magnetic techniques are known to have some sensitivity to stress, although magnetic measurements are usually also affected by other material properties such as microstructure. A way of measuring stress in a steel plate is described in GB 2 278 450, this method using a probe containing an electromagnetic core to generate an alternating magnetic field in the plate, and then combining measurements from two sensors, one being a measure of stress-induced magnetic anisotropy, and the other being a measure of permeability. The probe is gradually turned around so the magnetic field has a plurality of different orientations in the plate, and these measurements are taken at each such orientation. The signals are affected not only by stress, but also by the lift off from the surface (i.e. the gap between the probe and the surface), and so must be corrected for lift off.